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M Nagatomo, 1995, "An Approach to Develop Space Solar Power as a New Energy System for Developing Countries", Solar Energy, Vol. 56, No 1, pp 111-118.
Also downloadable from http://www.spacefuture.com/archive/an approach to develop space solar power as a new energy system for developing countries.shtml

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An Approach to Develop Space Solar Power as a New Energy System for Develop Countries
Makoto Nagatomo
Abstract - The idea of space solar power proposed by Glaser was explained as a set of a solar power power stations in geostationary earth orbit to transmit microwave power and a ground station to receive the microwave power. Most of the ideas and concepts since Glaser used the same context. On the other hand, Collins et al (Proceedings SPS '91, pp.132-141, 1991) introduced the concept of microwave "fuel" to assess the commercial relations of power from space, in which space solar power stations are considered to sell microwave power to any unspecified rectenna. This concept changed the theoretical context of "power from space" to an industrial and economic relation of producers and buyers of an industrial product. This new context has been applied to the SPS 2000 conceptual study. As a result, if 2.45 GHz microwave power transmission is used, each rectenna can be planned and engineered independently from the space sector by local users, especially in developing countries, who are familiar with such activities as introducing solar energy systems.
1. INTRODUCTION

Glaser's original idea (1968) was to build a photovoltaic solar power station in geostationary earth orbit, which converts generated electric power to microwaves to be beamed to a receiving antenna on the earth where the microwaves are rectified and transformed to utility power for public use. This idea of Glaser was followed by the concept of the SPS Reference System designed by NASA and the contractors for the SPS Concept Development and Evaluation Program (CDEP). Recently general features of this program were summarized by Koomanoff and Bloomquist (1993). In this paper, SPS is an abbreviation for Satellite Power System according to that study. In the CDEP, SPS was assumed to be the U.S. national power system to supply 300 GW electric power. The study concluded positively that research and development of SPS should proceed. However, the study was interrupted and no follow-on action has been taken to date.

The idea of space solar power satellites is revolutionary as it converts the limitless solar energy into high quality sustainable energy for humankind. In addition to this, the idea indicates a direction for productive space activities which would be able to recover the huge investments in early space exploration and technology development for spaceflight by supplying part of the world electricity market which is many times larger than the space industry. Thus this new system was expected to interest the two parties of energy development and space activities. However, both parties have been indifferent to SPS. Obviously the SPS system envisioned in these pioneering works was too unique to be accepted by society as it was proposed.

A quarter of a century has passed since the first publication concerning power from space. While little progress of research and development for solar power satellites has been observed for more than a decade, concern about the future of global energy and the environment is growing. I would like to examine the feasibility of space solar power systems finding a new context for research and development that can be expected to lead to industrial activities for new energy development as soon as possible.

2. FEATURES OF PAST SPS RESEARCH

After Glaser, many concepts of power systems using space solar energy have been proposed, mainly by planners of space projects. They stressed various aspects of solar power from space. Some discussed the system design of the space section of the power system. Others discussed specific technical areas such as the method of power conversion, power transmission and reception. These efforts were devoted to filling the gap between the Reference System and present-day space technology from various viewpoints such as demonstration of principle, early phase development, alternate system configurations and different application from power from space. I have summarized these efforts in two categories in the following.

2.1. Ideas and concepts

One of the biggest issues is concerned with the locations of solar power stations and customers. Glaser's original idea and the Reference System chose geostationary orbit ( GEO) for space solar power stations. Both systems consisted of solar panel(s) actively controlled to face the sun and transmitting antenna(s) beaming microwaves power to power receiving stations on the earth. In this case, the positions of the space power stations relative to the receiving stations can be practically fixed, so that these power stations serve exclusively for the rectennas just like power stations built in the territory of the United States. However, the distance of power transmission is very long, more than 36,000 km, that is the height of the orbit above the equator.

Many derivative ideas and concepts proposing to use low earth orbits ( LEO) instead of GEO are intended to avoid the difficulty of wireless power transmission over this long distance, and also to ease the difficulty of transportation of materials from earth. In Fig. 1, a summary of proposed locations of space solar power stations and receiving stations is schematically shown by arrows. The arrow directions indicate the direction of power transmission. As seen in this figure, some space power systems are intended to beam power not to the ground but to other space systems in space. I would like to focus my discussion on power from space for terrestrial use, because we will have the same economic scale to evaluate the competitiveness of various SPS proposals in comparison with solar power systems to be built on the earth. Then three space power station's locations, GEO, Moon and LEO are identified in Fig. 1.

Fig. 1. Summary of past ideas and concepts of SPS featured by design factors: where, who and when. The arrows indicate directions of wireless power transmission from the locations of solar power stations on the moon, GEO (geostationary orbits) or LEO (low earth orbits) to users' locations of S/C (spacecraft), GPO, LEO or the earth.

" GEO to Earth" is represented by the Reference System which was intensively studied to conceptualize a system to be used as a reference for evaluation. "Moon to Earth" has been discussed for the purpose of utilizing lunar resources to reduce transportation of materials from earth. The key issue of this study is to solve a planetary-scale economic problem to come in the future. On the other hand, " LEO to Earth" is a compromise to ease the difficulty of the " GEO to Earth" approach for which technical and economic risks are predicted. There have been several studies of the " LEO to Earth" approach. Typical orbits studied were an equatorial orbit and a Molniya type of orbit. (The Molniya is a Russian satellite communication system using satellites in a highly eccentric orbit for relaying radio communication between two ground stations.) These studies mainly focussed on the space solar power stations and little or not at all on the ground receiving stations.

2.2. Technology options

Technology to be used for SPS has been proposed mostly as a part of each study of SPS concepts, or as research plans such as the SEEL proposed by Kuriki et al. (1985). Most of the past conceptual studies on SPS have been conducted by space researchers and planners on an individual basis in connection with specific technical fields of satellite subsystems as follows.

2.2.1. Large space structure.

The Reference System consisted of 60 5 GW solar power stations. A typical 5 GW solar panel was 5 km wide and 10 km long and deployed on a large structure 1 km in height made of truss beams. The size of the transmitting antennas was 1 km in dia, given as a mission requirement of microwave power transmission which specified a microwave frequency of 2.45 6Hz, and the maximum microwave power density of 23 mW/cm2 in the ionosphere. This large structure has been a technical field symbolizing the space solar power stations, and related technology options were proposed.

2.2.2. Power transmission.

To reduce the large size of transmitting antennas of the Reference System, many researchers proposed to use higher frequencies; 5 and 9 6Hz. Some proposed to use lasers for power transmission, and even solar power conversion with lasers. In principle, any type of radio and light waves can be used for wireless power transmission for SPS, if it is practical. However, most proposed technologies have been scientific and have ignored such engineering problems as the thermal and structural design of antennas.

2.2.3. Power generation.

For the Reference System, photovoltaic cells were chosen to convert solar energy into electricity. Thermal-mechanical generators which are supposed to be a more mature technology than the previous two fields, were not employed because there is no past experience of space use. By this criterion, laser power generation would be as premature as nuclear fusion.

2.2.4. Control technology.

Aerospace companies designed each solar power station of the SPS Reference System as a huge spacecraft whose mission was to beam microwave power to ground based antennas. Attitude control of such a large space system has been a challenge for space engineers. Interactions of such a large electric system moving in the space environment have interested space scientists who have been involved in environmental issues of SPS programs.

In summary, past research on SPS has not been systematically coordinated, but has been done voluntarily by space engineers, and has involved few engineers from the power industry. Results of studies were rarely evaluated from the standpoint of being energy systems for terrestrial use.

3. REALITY OF NEW ENERGY DEVELOPMENT

Economy and environment are the key words of the new energy situation. As well known by the global warming issue, recent energy development is concerned not only with energy to support economic growth but also with environment protection. In the New Earth 21 Action Program proposed by the Japanese government to the international community, space solar power generation is categorized together with nuclear fusion as a candidate for development as a future energy technology until 2040. It is also stated that "The lasting solution to global warming thus requires an undertaking within a framework of international cooperation, not only among the developed countries but also developing countries (Hashimoto, 1991)".

Although it is not clear if the general scheme of the New Earth 21 Action Program was accepted by other governments, the view is encouraging to space solar power research, and suggests that the general plan of research for space solar power should be prepared on a realistic basis of current energy developments rather than as a theoretical option for future space programs. From this viewpoint, I summarize two aspects of realism required for planning space solar power research and development as follows.

3.1. Sustainable energy

To meet the final goal of providing sustainable energy for future growth and protection of the environment, the design and technology for space solar power should be evaluated by the criteria of availability of resources, energy economy (payback time) and waste production such as carbon-dioxide through all the processes required for production of SPS. Power from space should be competitive with other energy sources in this respect.

For example, we should consider long-term factors of solar cells to be used for SPS Solar cells are still a big target of sustainable energy for which industry is undertaking research and development to increase the various performances and production. It should be noted that the present annual production in the world is several 10 MW, and only a fraction of 5 GW would be the maximum capacity of gallium arsenide (Ga-As) solar cells produced by all resources available on the earth. Thus, the Ga-As Reference System of 60 5 GW solar power stations for the United States early next century is found to be based on unrealistic assumptions.

Generally production costs broadly reflect energy consumption for industrial products, and terrestrial use solar cells are now reaching the level to compete with other energy systems on this factor. The energy payback time is defined as the time required for a power system to recover the total energy used for its production. In the case of terrestrial solar panels, the payback time including the supporting structure is estimated to be less than 30 yr. These efforts for terrestrial solar cells are acknowledged as a firm basis to support efforts for space solar power, because the most important advantage of space solar power over terrestrial solar power is in the fact that for a similar solar cell panel approximately one order of magnittide more solar energy is available in space than on the earth. Therefore, if solar cells are used for SPS, SPS will be designed as a variation of a solar power station on earth in terms of solar cell technology.

3.2. Economy

Economy is the fundamental motivation for people to accept a new system in society. When a new system like a space solar power system is introduced in society, finance is the key factor. In the past the space solar power system was a power system which was a large collection of electric power systems to be deployed over a huge territory from the earth to space. As a result, the project was predicted to be so big that it was unrealistic for interested parties, except governments, to invest in it.

The concept of microwave "fuel" (Collins, 1991) is unique, as it broke down the traditional SPS into the relation of suppliers and buyers of microwave "fuel". Using this concept, space solar power stations sell microwave power to any unspecified rectenna, so that commercial relations can be established between them. This concept not only gives a new assessment method of this new power system, but it is also expected to establish definite goals for technology development for orbital power stations and for ground power receiving stations independently.

Transportation to and from space has been considered to be the main obstacle to development of massive systems like space solar power stations in space. In principle, this can be improved technically by designing reusable space vehicles to be operated similarly to present-day airlines. For such a space transportation system, the businesses building and operating space power stations will be major customers. The economy of microwave "fuel" will be important in assessing this respect as well.

4. PRACTICAL APPROACHES TO SPACE SOLAR POWER RESEARCH

In addition to having realistic targets, space solar power research should be practical, not theoretical. Assuming that we were to prepare a business plan for space solar power research, I would like to emphasize familiarity as the most important feature to be required.

The first is familiar technology available from the industries concerned. It will be most favorable for industry to participate in the project, but even otherwise, the necessary technology should be available on a commercial basis. Space technology is not attractive to commercial industries, because it is too expensive and its markets are too small.

The second is familiar size of activities which may be represented by ordinary research and development prqjects of industry and governments. Research on a new energy system is the first opportunity for future customers and developers to be familiarized with it. It is important to make it attractive and beneficial to potential partners even if some risks are involved.

The third is the promise of low cost space transportation. Space is unknown territory for ordinary people. Even highly educated people believe that space is only for adventurers and not for business. The existing commercial launch business is desperate in this respect, but fortunately development of SSTO (single-stage-to-orbit) vehicles has been started in the United States, and transportation cost is expected to be reduced to an acceptable level.

4.1. SPS 2000 study

The SPS 2000 study is a design study conducted by researchers of the Solar Power Satellite Working Group led by the Institute of Space and Astronautical Science of Japan. The main purpose of the study was to identify study subjects in each special field through developing a strawman concept of SPS The study was intended to familiarize them with solar power satellites, and the concept was intended to be practical in economy and realistic in technology. The two substantial features of the economic aspect at the earlier stage are a lower power level and a shorter transmission distance. A lower power level is practical for reducing the risk of marketing the produced power. A shorter transmission distance means firstly reduction in the size of antennas, especially of the microwave transmission antenna onboard the space power station, and secondly lower altitude of orbit for the solar power station to be transported by rockets, and to be constructed. In the study of SPS 2000, the solar power station was assumed to be built at an altitude of 1100 km above the equator. Different from the Reference System, it moves eastwards with a period of about 90 mm. Accordingly, the users of this power system have to get the microwaves beamed to their receiving facility during the power station passes over the rectenna. The microwave power is only 10 MW. More detail on the concept of SPS 2000 was reported by Nagatomo et al (1994).

Fig 2. SPS 2000 system functional model displayed at Institute of Space and Astronautical Science on 30 July 1994. Four transmitting antenna elements designed as required for the actual satellite and ninety six rectenna elements shown in Fig. 3 have been built into the satellite model (upper) and rectenna model (lower level), respectively

The SPS 2000 study was conducted on a voluntary basis with support by the organizations to which researchers belong. After the conceptual study ended with submission of the final report on a preliminary design of a proposed system, the researchers are continuing experimental studies on individual technical fields. Some of the results have been applied for assembling a demonstration model displayed at ISAS in Sagamihara in July 1994 as shown in Fig. 2, and later at the exhibition on energies for today and tomorrow held from December 1994 at the Stella Matutina Museum in Reunion, France.

An interesting feature of the SPS 2000 study is that because this study has not been limited to the initial framework of study, that is to design a strawman solar power satellite, it is escalating to research on the rectenna sites. A 1994FY Ministry of Education, Science and Culture Research Grant was awarded for these studies.

4.2. The key issue of ground receiving station: economy

Among many studies on SPS systems, there are relatively few papers on subjects relating to rectennas. Most of the papers discussed the ground receiving station as a part of the total system in terms of radio power transmission, or focussed on electric technology to be used for microwave power rectification. In the case of the U.S. CDEP, the SPS was assumed to be the U.S. national electric power system. The study on the rectenna was technical concerning processing the high power microwaves from space for utility power with the same standard of large power plants presently operated in industrially developed countries. Accordingly, little attention was paid to the commercial aspect of the rectennas which were assumed to be given the huge market of electric power together with nation-wide grid networks.

The study for CDEP is not applicable to rectennas of the SPS 2000 concept, whose power level is much smaller and premature in its quality of electricity. The microwave power available for reception by a rectenna at one time is only 10 MW for about 5 mm, or averaged power of 300 kW if storage is provided. By the standards of industrialized nations, this small and intermittent power supply would seem to limit the practicability of this system as an electric power system. However, from the standpoint of solar energy development, for example, solar photovoltaic cells, 300 kW will be large enough to be designed as a regional electric power system. Even several-kilowatt power systems are being tested and verified for practical use of electricity. It should be also noted that the first commercial power station built by Edison in the last century had a similar power generation capacity.

Current research on the rectenna has revealed a new area of study demand for SPS in developing countries. Energy in most such countries depends on wood and solar heat energy, which can be supplied by even such a small SPS as SPS 2000. In many places in Asia, small electric machines can free women and children from labor carrying water for their living. There is a demand, but we are still concerned about the practicality of SPS from the users' viewpoint, for example, construction and maintenance.

The construction and maintenance work required for the rectenna and relating facilities have to be compatible with workers' skill at the sites. No special high technology is used for critical parts of the rectenna and related equipment. All the system should be planned and managed locally and autonomously, as is done for ordinary power system installations. One of the technical outputs of the SPS 2000 study is a hand-built rectenna unit shown in Fig. 3. The total cost for the materials and electric parts is less than twenty Japanese Yen (about twenty U.S. cents), which is mainly for the diode. Although the output power of the unit shown in Fig. 3 is 1.3 mW, fundamental technology is included in this model. The unit shown in Fig. 3 was made by Pignolet (1995).

5. RECOMMENDED MICROWAVE POWER TRANSMISSION

As a result of technical research based on the SPS 2000 conceptual study, wireless power transmission at 2.45 GHz is found to be the most practical method that can be applied for the early stage of space solar power stations. Although various ideas of wireless power transmission have been proposed for future systems, only 2.45 GHz can be recommended to be standardized as a practical standard for the following reasons.

5.1. Industrial targets of transmitting antenna design

Various concepts of SPS have been a mixture of theoretical and practical approaches intended to build SPS or to realize sustainable energy with space solar power. However, if the topics are focussed on the practical design of a transmitting antenna on a solar power station, only realistic technology should be used for design practice acceptable on an industrial basis. In this respect, selection of microwaves of 2.45 GHz in the original idea of Glaser and the Reference System has been a very thoughtful insight to the future and present needs. The biggest issue accompanying the use of higher frequencies is thermal design of the antenna to dissipate the heat generated by microwave generators due to conversion losses. For the Reference System which used high efficiency klystrons as microwave amplifiers, the heat was dissipated from the antenna surface without special thermal radiators. If high frequency microwaves are used in a similar design, additional panels for heat dissipation will be required. Another issue is the availability of industrial products for antenna design. Whether tubes or semiconductors are selected, available parts and components are produced according to the requirements of the communications industry. Only recently industry has been interested in semiconductor technology of 2.45 GHz.

Fig. 3. An element of an actual rectenna of SPS 2000 featured by a H-shaped antenna and a wire mesh reflector.

Improvement of conversion efficiency and cost is expected to meet the requirements of SPS relatively soon compared with higher frequency devices.

5.2. Autonomy of rectenna research

To develop demand for power from space, planning of reetennas for regional purposes by residents is essential. Standardization of microwave power transmission will be the basic, technical interface between the users and power suppliers in this case, just like the present standard of commercial power lines specified by voltage and alternating current frequency. To be analogous to this, if microwave transmission is standardized in frequency, power density and other parameters, more people will participate in the study of space solar power and as a result will support the activities to realize it. The only requirement in this ease is that the antenna technology should not be too high to be managed by ordinary people in developing countries. Experience of rectenna design for SPS 2000 indicated that the antenna elements for 2.45 GHz microwaves can be hand-built and the cost is reasonably low.

5.3. Interference with radio communications

With the growth of the personal communication industry, allocation of more radio frequency bands are being requested. The 2.4-2.5 GHz ISM band now allotted to industrial uses such as heating and cooking is already in the range of communication frequencies. There are discussions on reallocation of this band to communications in connection with concerns about interference of high power microwaves from space solar power station. However, according to the past experience of industrial application of 2.45 GHz, and the forecast of planned research on wireless power transmission, the 2.4-2.5 GHz band should be reserved for power industry use, unless other solutions are found.

6. CONCLUSIONS
  1. There are many concepts of space solar power systems that have been proposed for space solar energy to be used for humankind. However, most of them were theoretical and not evaluated on the basis of becoming practical power systems. The SPS 2000 study was made on practical assumptions and has indicated a realistic approach to space solar power research which can be interpreted as follows:
  2. To facilitate research on this power system as a future energy source to compete with other sustainable energy candidates, it is necessary to consider the space solar power system as a variation of solar power systems now under research and development for terrestrial use.
  3. The advantage of space solar power over terrestrial solar systems is one order of magnitude larger solar power in space than on the earth. The disadvantage is the high cost of transportation of the required facilities to space. Even if reusable space transportation systems under development realize lower costs, the advantage over terrestrial systems is expected to be marginal. A cost target is therefore mandatory for engineering space solar power stations. The microwave "fuel" concept can be applied to this case too.
  4. It is practical to apply the concept of microwave "fuel" as the interface between space power suppliers and buyers, as utility power suppliers and consumers are related to each other by the standard of commercial electric power. Considering that a properly selected microwave frequency makes it possible for users to plan and even build their rectennas, I strongly recommend the use of 2.45 GHz as a standard for wireless power transmission.
REFERENCES
  1. P Q Collins and R Tomkins, 1991, " A method for utilities to assess the SPS commercially", Proc. SPS'91 pp.132-141, SEE (1991)
  2. P F Glaser, " Power from the Sun: Its Future", Science Magazine 162, 857-866(1968)
  3. M Hashimoto, 1991, " New Earth 21 Action Program", Presented at SPS '91 (in English), Original article by a MITI office in Tsu-San Journal, November 1990, pp.22-25 (in Japanese)
  4. F A Koomanoff and C E Bloomquist, 1993, " Solar Power Satellites [1]", pp.26-59. Horwood London
  5. K Kuriki, M Nagatomo and T Obayashi, 1985, " Space Energetics and Environment Laboratory (SEEL)", Space Solar Power Rev. 5, 197-205
  6. M Nagatomo, S Sasaki and Y Naruo, 1994, "Conceptual study of a solar power satellite, SPS 2000", Proc. 19th ISTS, Yokohama, pp 469-476, Agne
  7. G Pignolet, 1995, " Design of a low-power rectenna for a low-cost SPS-2000/WPT demonstration model", ISAS Note No.573, Institute of Space and Astronautical Science
M Nagatomo, 1995, "An Approach to Develop Space Solar Power as a New Energy System for Developing Countries", Solar Energy, Vol. 56, No 1, pp 111-118.
Also downloadable from http://www.spacefuture.com/archive/an approach to develop space solar power as a new energy system for developing countries.shtml

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